It has been recognized for some time that zinc ions derived from zinc compounds employed in dentifrices, oral rinses, mouthwashes and the like have the beneficial effect of reduction of calculus formation and undesirable mouth odor. Zinc chloride has been used for many years, but it is very astringent and has an unpleasant taste that is difficult to mask. In a mouthwash or oral rinse formulation zinc chloride has a pH of about 3.0 which accounts for its high level of astringency.
Insoluble zinc salts such as zinc citrate and zinc carbonate have been used in dentifrice compositions (Pasternak U.S. Pat. No. 1,861,189 and Bley U.S. Pat. No. 1,943,467). Moreover, zinc citrate has been used for periodontal treatment and in dentifrices as a calculus and plaque control agent. Pader in U.S. Pat. No. 4,100,269 has claimed a long list of insoluble zinc compounds all of which do not provide a sufficient amount of zinc ions because of poor solubility.
In U.S. Pat. No. 4,289,755 (Dhabhar) insoluble zinc citrate is employed and solvated by means of an excess of citric acid in an attempt to overcome the relative insolubility problem of zinc compounds. U.S. Pat. No. 4,289,754, also issued to Dhabhar, employs sodium or ammonium zinc citrate as a novel soluble zinc compound, partially overcoming the problem of solubility discussed in U.S. Pat. No. 4,289,755. However, the solubility of sodium zinc citrate is given as only 1.17 g/100 ml, thus precluding a concentration of the zinc salt of more than about 1.2%. In U.S. Pat. No. 4,469,674, zinc salicylate is employed as a source of zinc ions. As set forth above, due to insolubility of the zinc citrate it is impossible to formulate high concentrations of zinc ions. The sodium zinc citrate of U.S. Pat. No. 4,325,939 is stated to have a solubility of about 1.2%, which, while greater than that of zinc citrate, still falls short of providing a desirable concentration of zinc ions.